IMPROVEMENT OF THERMAL SPALLING RESISTANCE OF ALUMINA-GRAPHITE MATERIALS BY NANO-TECHNOLOGY

摘要

Ladle shrouds and nozzle refractories utilized in the final stage of continuous casting must exhibit properties such as molten steel transportation, air shielding, flow control, and inclusion elimination. Sometimes the ladle shrouds, in particular, are subjected to difficult service conditions such as casting without preheating where fluctuations in temperature are severe. Therefore, excellent thermal spalling resistance and durability are needed for top performance. To fulfill these requirements, the composition of the ladle shrouds, normally Al_2O_3-Graphite or Al_2O_3-SiO_2-Graphite (here below referred to as "AG materials"), has been adjusted to meet customer's expectations. There are two main methods known to improve the thermal spalling resistance of AG materials: 1) the increase of fused silica or graphite; and 2) the adjustment of the grain size in the aggregate. However, both of these methods have shortcomings. Adding fused silica or graphite improves thermal spalling resistance but often decreases corrosion resistance to molten steel and results in a decrease of durability. Adjusting grain size often causes properties of the shroud materials to change during casting and can lead to interruption of the casting operation. Accordingly it is considered essential to improve thermal spalling resistance not by increasing fused silica or graphite and also not by adjusting the grain size of aggregate. The carbon matrix of a ladle shroud is studded three-dimensionally, thereby surrounding aggregate such as Al_2O_3 or SiO_2 in AG material. Therefore, the properties of the carbon matrix may affect thermal spalling resistance. The effect is evaluated in mixes of constant chemical composition and constant aggregate grain size, and the results are reported in this paper. As the concrete method, unique resin, which characteristically forms a nano-dimensional structure of carbon fiber in the process producing solid carbon, is added to the AG material to evaluate the change in thermal spalling resistance. Results confirm that the unique resin decreases thermal expansion, decreases elasticity and increases strength and, therefore, is very effective to improve thermal spalling resistance. By adding the unique resin, thermal spalling resistance is greatly improved in AG materials. Further improvement is carried out by decreasing the amount of aggregates less resistant to corrosion such as silica and graphite. When the resulting AG materials are used in actual service, corrosion resistance and thermal spalling resistance are superior to that of conventional mixes.